Photon-counting x-ray detector system having an adaptive anti-coincidence system

ABSTRACT

There is provided a photon-counting x-ray detector system ( 200 ) comprising a plurality of photon-counting channels ( 220 ), and at least one anti-coincidence circuit ( 230 ), each of which is connected to least two of the channels and configured to detect coincident events in the connected channels. The x-ray detector system ( 200 ) further comprises an anti-coincidence controller ( 240 ) configured to control the operation of said at least one anti-coincidence circuit based on photon count information by gradually adapting the operation of said at least one anti-coincidence circuit with increasing count rates, starting from a threshold count rate.

TECHNICAL FIELD

The present invention generally relates to x-ray imaging and x-raydetector systems, and more particularly to a photon-counting x-raydetector system, an anti-coincidence system for a photon-counting x-raydetector system as well as a controller for such an anti-coincidencesystem and a corresponding computer-program product.

BACKGROUND

Radiographic imaging such as x-ray imaging has been used for years inmedical applications and for non-destructive testing.

Normally, an x-ray imaging system includes an x-ray source and an x-raydetector system. The x-ray source emits x-rays, which pass through asubject or object to be imaged and are then registered by the x-raydetector system. Since some materials absorb a larger fraction of thex-rays than others, an image is formed of the subject or object. Thex-ray detector may be of different types, including energy-integratingdetectors and photon-counting detectors.

Performance of x-ray imaging detectors are commonly measured using thedetective quantum efficiency (DQE). The DQE is defined as the squaredsignal to noise in the output from the detection system divided by thesquared signal-to-noise ratio of the input to the detector, i.e. dividedby the squared signal-to-noise ratio that would be measured by an idealdetector. The DQE is a function of spatial frequency in the image.Higher DQE corresponds to better detector performance and less noise inthe measured image.

A problem in photon-counting x-ray imaging is that a single photon maycause a pulse to be counted in more than one detector element, sometimesalso referred to as a detector pixel or simply pixel, as will beexplained later on. This can be caused by several mechanisms. One suchmechanism is charge sharing, where the charge cloud generated by aphoton interaction is collected by more than one electrode. Another suchmechanism is Compton scatter, which causes a photon to deposit energy ina first pixel and then propagate to a second pixel and deposit moreenergy there. A third mechanism that can cause double counting isfluorescence where an original x-ray photon interaction in a first pixelleaves an inner electron shell of an atom in an excited state, which issubsequently de-excited by the emission of a fluorescence photon, whichis reabsorbed in a second pixel. This means that a fraction of theevents are counted twice, and since this happens randomly, it degradesthe DQE of the detector, thereby giving increased image noise.Furthermore, double-counting of photons can cause blurring of the imageand degrade energy resolution. It is thus an objective to register eachphoton only once, with the correct photon energy and in the originalpixel of interaction.

To achieve this objective, it may be beneficial to implementanti-coincidence logic in the x-ray detector. This anti-coincidencelogic can detect simultaneous events and ensure that simultaneous pulsescaused by the same photon is only counted once. Such schemes mayfurthermore be refined so that they use the information contained in theregistered set of pulse heights to estimate the original position ofinteraction and the original photon energy.

A problem with anti-coincidence logic schemes is that they may identifypulses generated by two photons arriving close to each other in timeincorrectly, as generated by a single photon. This is called falsecoincidence, as opposed to true coincidence which is when theanti-coincidence logic correctly identifies two pulses as generated bythe same original photon.

False coincidence causes loss of counts and therefore degrades DQE andincreases image noise. Furthermore, false coincidence can distort theenergy information if the energies of the coincident photons are summedtogether. If the probability of false coincidence is large enough, thedetrimental effect of false coincidence may outweigh the benefits ofphoton counting.

For a photon-counting detector to be useful in specific applications,such as Computed Tomography (CT), the detector must be able to handlethe count rates occurring in the application.

U.S. Pat. No. 6,559,453 relates to a method of enhancing contrastinformation in x-ray imaging wherein the signals from the photons aregiven a weight that is influenced by the possibility of charge sharingbetween adjacent sensor elements.

U.S. Pat. No. 7,214,944 relates to a radiation detection device whichcompares the temporal overlap of signals from different detectorelements, with the objective of making it possible to distinguish realevents from false events at high count rates.

U.S. Pat. No. 7,473,902 relate to a method for taking radiographs wherecharge pulses of bordering pixel units are added together to a totalcharge pulse.

U.S. Pat. No. 8,050,385 relates to a coincidence detection unit withparameters and thresholds that may have to be adjusted such that theadvantage from detecting coincidences is greater than the disadvantagefrom incorrectly removing false double counts.

U.S. Pat. No. 9,031,197 relates to a method for detecting truecoincidence of charge pulses, by allocating the height of the pulses toone of several intervals and analyzing the combination of allocations tointervals in adjacent picture elements.

The publication T. Koenig et al. “Charge Summing in Spectroscopic X-RayDetectors with High-Z Sensors”, IEEE Transactions on Nuclear Science 60(6), pp. 4713-4718, 2013, relates to an anti-coincidence logicimplementation based on the summation of collected charge in adjacentpixels. This anti-coincidence logic gives improved reconstruction of theincident energy spectrum at low photon fluxes, but causes severe countloss at higher fluxes, above 5·10⁶ counts/mm²·s.

U.S. Pat. No. 9,207,332 relates to an x-ray detector with a low-fluxmode where charges collected by neighboring pixels are summed togetherbefore being digitized by comparators, and a high-flux mode where nosumming of charges from neighboring pixels is made before the signal isdigitized by the comparators, but the resulting counts in neighboringpixels are summed together after digitization.

US Patent Application 20160282476A1 relates to an x-ray detector withtwo counting modes, which initially measures a first count in a firstcounting mode and, based upon this count value measures a second countvalue in a second counting mode.

There is however still a need for a detector with improvedanti-coincidence logic which gives good image quality both for low andhigh incident photon flux.

SUMMARY

It is thus a general object to provide improved anti-coincidence logicfor a photon-counting x-ray detector system.

It is a specific object to provide a photon-counting x-ray detectorsystem.

Another object is to provide an anti-coincidence system for aphoton-counting x-ray detector system.

Yet another object is to provide a controller for an anti-coincidencesystem of a photon-counting x-ray detector system.

Still another object is to provide a corresponding computer-programproduct.

These and other objects are met by embodiments of the proposedtechnology.

According to a first aspect there is provided a photon-counting x-raydetector system comprising a plurality of photon-counting channels, andat least one anti-coincidence circuit, each of which is connected toleast two of the channels and configured to detect coincident events inthe connected channels. The x-ray detector system further comprises ananti-coincidence controller configured to control the operation of saidat least one anti-coincidence circuit based on photon count informationby gradually adapting the operation of said at least oneanti-coincidence circuit with increasing count rates, starting from athreshold count rate.

In this way, the proposed technology provides good image qualityindependent of the incident photon flux rates. The inventors haverecognized that abrupt changes of the operation of the anti-coincidencecircuit(s) based on measured count rates may cause artifacts in thex-ray images, and that a gradual adaptation of the operation of theanti-coincidence circuit(s) with increasing count rates eliminates or atleast reduces such artifacts in the images.

In particular, the proposed technology makes it possible to graduallylimit the influence of the anti-coincidence circuit(s) with increasingcount rates, e.g. to provide a smooth count rate characteristics,thereby avoiding image artifacts.

According to a second aspect there is provided an anti-coincidencesystem for a photon-counting x-ray detector system having a plurality ofphoton-counting readout channels, wherein the anti-coincidence systemcomprises at least one anti-coincidence circuit, each of which isconnected to least two of the channels and configured to detectcoincident events in the connected channels. The anti-coincidence systemfurther comprises an anti-coincidence controller configured to controlthe operation of said at least one anti-coincidence circuit based onphoton count information by gradually adapting the operation of said atleast one anti-coincidence circuit with increasing count rates, startingfrom a threshold count rate.

According to a third aspect there is provided a controller for ananti-coincidence system of a photon-counting x-ray detector systemhaving a plurality of photon-counting readout channels, wherein theanti-coincidence system comprises at least one anti-coincidence circuit,each of which is connected to least two of the channels and configuredto detect coincident events in the connected channels. The controller isconfigured to control the operation of said at least oneanti-coincidence circuit based on photon count information by graduallyadapting the operation of said at least one anti-coincidence circuitwith increasing count rates, starting from a threshold count rate.

According to a fourth aspect there is provided a computer-programproduct comprising a computer-readable medium having stored thereon acomputer program for controlling, when executed by a processor, ananti-coincidence system of a photon-counting x-ray detector systemhaving a plurality of photon-counting readout channels. Theanti-coincidence system comprises at least one anti-coincidence circuit,each of which is connected to least two of the channels and configuredto detect coincident events in the connected channels. The computerprogram comprises instructions, which when executed by the processor,cause the processor to control the operation of said at least oneanti-coincidence circuit based on photon count information by graduallyadapting the operation of said at least one anti-coincidence circuitwith increasing count rates, starting from a threshold count rate.

Other advantages will be appreciated when reading the detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments, together with further objects and advantages thereof,may best be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating an example of an overallx-ray imaging system.

FIG. 2 is a schematic diagram illustrating another example of an x-rayimaging system.

FIG. 3 is a schematic diagram illustrating examples of the energyspectrum for three different x-ray tube voltages.

FIG. 4 is a schematic diagram illustrating an example of a photoncounting mechanism.

FIG. 5 is a schematic diagram of an X-ray detector according to anexemplary embodiment.

FIG. 6 is a schematic diagram illustrating an example of a semiconductordetector module according to an exemplary embodiment.

FIG. 7 is a schematic diagram illustrating an example of semiconductordetector module according to another exemplary embodiment.

FIG. 8 is a schematic diagram illustrating an example of a semiconductordetector module according to yet another exemplary embodiment.

FIG. 9 is a schematic diagram, illustrating an example of aphoton-counting x-ray detector system having a plurality ofphoton-counting channels and at least one anti-coincidence circuit andan associated anti-coincidence controller according to an embodiment.

FIG. 10 is a schematic drawing illustrating another example of ananti-coincidence system implemented in a photon-counting x-ray detectorsystem.

FIG. 11A-C are schematic curve diagrams showing examples of count-ratecharacteristics of detectors with and without anti-coincidence logic,and with the proposed rate-dependent anti-coincidence logic.

FIG. 12 is a schematic diagram illustrating an example of acomputer-implementation according to an embodiment.

DETAILED DESCRIPTION

It may be useful to begin with a brief overview of an illustrativeoverall x-ray imaging system, with reference to FIG. 1. In thisnon-limiting example, the x-ray imaging system 100 basically comprisesan x-ray source 10, an x-ray detector system 20 and an associated imageprocessing device 30. In general, the x-ray detector system 20 isconfigured for registering radiation from the x-ray source 10 that mayhave been focused by optional x-ray optics and passed an object orsubject or part thereof. The x-ray detector system 20 is connectable tothe image processing device 30 via suitable analog processing andread-out electronics (which may be integrated in the x-ray detectorsystem 20) to enable image processing and/or image reconstruction by theimage processing device 30.

As illustrated in FIG. 2, another example of an x-ray imaging system 100comprises an x-ray source 10, which emits x-rays; an x-ray detectorsystem 20, which detects the x-rays after they have passed through theobject; analog processing circuitry 25, which processes the rawelectrical signal from the detector and digitizes it; digital processingcircuitry 40 which may carry out further processing operations on themeasured data such as applying corrections, storing it temporarily, orfiltering; and a computer 50 which stores the processed data and mayperform further post-processing and/or image reconstruction.

The overall detector may be regarded as the x-ray detector system 20, orthe x-ray detector system 20 combined with the associated analogprocessing circuitry 25.

The digital part including the digital processing circuitry 40 and/orthe computer 50 may be regarded as a digital image processing system 30,which performs image reconstruction based on the image data from thex-ray detector. The image processing system 30 may thus be seen as thecomputer 50, or alternatively the combined system of the digitalprocessing circuitry 40 and the computer 50, or possibly the digitalprocessing circuitry 40 by itself if the digital processing circuitry isfurther specialized also for image processing and/or reconstruction.

An example of a commonly used x-ray imaging system is a ComputedTomography (CT) system, which may include an x-ray source that producesa fan or cone beam of x-rays and an opposing x-ray detector system forregistering the fraction of x-rays that are transmitted through apatient or object. The x-ray source and detector system are normallymounted in a gantry that rotates around the imaged object.

Accordingly, the x-ray source 10 and the x-ray detector system 20illustrated in FIG. 2 may thus be arranged as part of a CT system, e.g.mountable in a CT gantry.

A challenge for x-ray imaging detectors is to extract maximuminformation from the detected x-rays to provide input to an image of anobject or subject where the object or subject is depicted in terms ofdensity, composition and structure. It is still common to usefilm-screen as detector but most commonly the detectors today provide adigital image.

Modern x-ray detectors normally need to convert the incident x-rays intoelectrons, this typically takes place through photo absorption orthrough Compton interaction and the resulting electrons are usuallycreating secondary visible light until its energy is lost and this lightis in turn detected by a photo-sensitive material. There are alsodetectors, which are based on semiconductors and in this case theelectrons created by the x-ray are creating electric charge in terms ofelectron-hole pairs which are collected through an applied electricfield.

Conventional x-ray detectors are energy integrating, the contributionfrom each detected photon to the detected signal is thereforeproportional to its energy, and in conventional CT, measurements areacquired for a single energy distribution. The images produced by aconventional CT system therefore have a certain look, where differenttissues and materials show typical values in certain ranges.

Photon counting detectors have also emerged as a feasible alternative insome applications; currently those detectors are commercially availablemainly in mammography. The photon counting detectors have an advantagesince in principle the energy for each x-ray can be measured whichyields additional information about the composition of the object. Thisinformation can be used to increase the image quality and/or to decreasethe radiation dose.

Compared to the energy-integrating systems, photon-counting CT has thefollowing advantages. Firstly, electronic noise that is integrated intothe signal by the energy-integrating detectors can be rejected bysetting the lowest energy threshold above the noise floor in thephoton-counting detectors. Secondly, energy information can be extractedby the detector, which allows improving contrast-to-noise ratio byoptimal energy weighting and which also allows so-called material basisdecomposition, by which different materials and/or components in theexamined subject or object can be identified and quantified, to beimplemented effectively. Thirdly, more than two basis materials can beused which benefits decomposition techniques, such as K-edge imagingwhereby distribution of contrast agents, e.g. iodine or gadolinium, arequantitatively determined. Fourth, there is no detector afterglow,meaning that high angular resolution can be obtained. Last but notleast, higher spatial resolution can be achieved by using smaller pixelsize.

The most promising materials for photon-counting x-ray detectors arecadmium telluride (CdTe), cadmium zinc telluride (CZT) and silicon (Si).CdTe and CZT are employed in several photon-counting spectral CTprojects for the high absorption efficiency of high-energy x-rays usedin clinical CT. However, these projects are slowly progressing due toseveral drawbacks of CdTe/CZT. CdTe/CZT have low charge carriermobility, which causes severe pulse pileup at flux rates ten times lowerthan those encountered in clinical practice. One way to alleviate thisproblem is to decrease the pixel size, whereas it leads to increasedspectrum distortion as a result of charge sharing and K-escape. Also,CdTe/CZT suffer from charge trapping, which would lead to polarizationthat causes a rapid drop of the output count rate when the photon fluxreaches above a certain level.

In contrast, silicon has higher charge carrier mobility and is free fromthe problem of polarization. The mature manufacturing process andcomparably low cost are also its advantages. But silicon has limitationsthat CdTe/CZT does not have. Silicon sensors must accordingly be quitethick to compensate for its low stopping power. Typically, a siliconsensor needs a thickness of several centimeters to absorb most of theincident photons, whereas CdTe/CZT needs only several millimeters. Onthe other hand, the long attenuation path of silicon also makes itpossible to divide the detector into different depth segments, as willbe explained below. This in turn makes it possible for a silicon-basedphoton-counting detector to properly handle the high fluxes in CT.

When using simple semiconductor materials, such as silicon or germanium,Compton scattering causes many x-ray photons to convert from a highenergy to a low energy before conversion to electron-hole pairs in thedetector. This results in a large fraction of the x-ray photons,originally at a higher energy, producing much less electron-hole pairsthan expected, which in turn results in a substantial part of the photonflux appearing at the low end of the energy distribution. In order todetect as many of the x-ray photons as possible, it is thereforenecessary to detect as low energies as possible.

FIG. 3 is a schematic diagram illustrating examples of the energyspectrum for three different x-ray tube voltages. The energy spectrum isbuilt up by deposited energies from a mix of different types ofinteractions, including Compton events at the lower energy range andphotoelectric absorption events at the higher energy range.

A further development of x-ray imaging is energy-resolved x-ray imaging,also known as spectral x-ray imaging, where the x-ray transmission ismeasured for several different energy levels. This can be achieved byletting the source switch rapidly between two different emissionspectra, by using two or more x-ray sources emitting different x-rayspectra, or by using an energy-discriminating detector which measuresthe incoming radiation in two or more energy levels, also referred to asenergy bins.

In the following, a brief description of an example of anenergy-discriminating photon-counting detector is given with referenceto FIG. 4. In this example, each registered photon generates a currentpulse which is compared to a set of thresholds, thereby counting thenumber of photons incident in each of a number of energy bins.

In general, the x-ray photons, including also photons after Comptonscattering, are converted to electron-hole pairs inside thesemiconductor detector, where the number of electron-hole pairs isgenerally proportional to the photon energy. The electrons and holes arethen drifting towards the detector electrodes, then leaving thedetector. During this drift, the electrons and holes induce anelectrical current in the electrode, a current which may be measured,e.g. through a Charge Sensitive Amplifier (CSA), followed by a ShapingFilter (SF), as schematically illustrated in FIG. 4.

As the number of electrons and holes from one x-ray event isproportional to the x-ray energy, the total charge in one inducedcurrent pulse is proportional to this energy. The current pulse isamplified in the CSA and then filtered by the SF filter. By choosing anappropriate shaping time of the SF filter, the pulse amplitude afterfiltering is proportional to the total charge in the current pulse, andtherefore proportional to the x-ray energy. Following the SF filter, thepulse amplitude is measured by comparing its value with one or severalthreshold values (Thr) in one or more comparators (COMP), and countersare introduced by which the number of cases when a pulse is larger thanthe threshold value may be recorded. In this way it is possible to countand/or record the number of X-ray photons with an energy exceeding anenergy corresponding to respective threshold value (Thr) which has beendetected within a certain time frame.

When using several different threshold values, a so-calledenergy-discriminating detector is obtained, in which the detectedphotons can be sorted into energy bins corresponding to the variousthreshold values. Sometimes, this type of detector is also referred toas a multi-bin detector.

In general, the energy information allows for new kinds of images to becreated, where new information is available and image artifacts inherentto conventional technology can be removed.

In other words, for an energy-discriminating detector, the pulse heightsare compared to a number of programmable thresholds in the comparatorsand classified according to pulse-height, which in turn is proportionalto energy.

However, an inherent problem in any charge sensitive amplifier is thatit will add electronic noise to the detected current. In order to avoiddetecting noise instead of real x-ray photons, it is therefore importantto set the lowest threshold value (Thr) high enough so that the numberof times the noise value exceeds the threshold value is low enough notto disturb the detection of x-ray photons.

By setting the lowest threshold above the noise floor, electronic noise,which is the major obstacle in the reduction of radiation dose of thex-ray imaging systems, can be significantly reduced

In order to increase the absorption efficiency, the detector can bearranged edge-on, in which case the absorption depth can be chosen toany length and the detector can still be fully depleted without going tovery high voltages.

In particular, silicon has many advantages as detector material such ashigh purity and a low energy required for creation of charge carriers(electron-hole pairs) and a also a high mobility for these chargecarriers which means it will work even for high rates of x-rays. siliconis also readily available in large volumes.

The main challenge with silicon is its low atomic number and low densitywhich means it has to be made very thick for higher energies to be anefficient absorber. The low atomic number also means the fraction ofCompton scattered x-ray photons in the detector will dominate over thephoto-absorbed photons which will create problem with the scatteredphotons since they may induce signals in other pixels in the detectorwhich will be equivalent to noise in those pixels. Silicon has howeverbeen used successfully in applications with lower energy as is forexample outlined by M. Danielsson, H. Bornefalk, B. Cederström, V.Chmill, B. Hasegawa, M. Lundqvist, D. Nygren and T. Tabár,“Dose-efficient system for digital mammography”, Proc. SPIE, Physics ofMedical Imaging, vol. 3977, pp. 239-249 San Diego, 2000. One way toovercome the problem of low absorption efficiency for silicon is tosimply make it very thick, the silicon is produced in wafers which areapproximately 500 μm thick and these wafers can be oriented so that thex-rays are incident edge-on and the depth of silicon may be as much asthe diameter of the wafer if required.

Another method to make silicon deep enough to get high efficiency isadvocated in U.S. Pat. No. 5,889,313 of Sherwood Parker “Threedimensional architecture for solid state radiation detectors” 1999, thisis an inventive method but involves some non-standard production methodswhich may be the reason why it has not been used in commercial imagingdetectors.

The first mentioning of crystalline silicon strip detectors in edge-ongeometry as an x-ray detector we could find is R. Nowotny: “ApplicationOf Si-Microstrip-Detectors In Medicine And Structural Analysis” NuclearInstruments and Methods in Physics Research 226 (1984) 34-39. Itconcludes that silicon will work at low energies such as for breastimaging but not for higher energies such as computed tomography mainlybecause of the higher fraction of Compton scattering and problemsrelated to this.

The edge-on geometry for semiconductor detectors is also suggested inU.S. Pat. No. 4,937,453 of Robert Nelson “X-ray detector forradiographic imaging”, U.S. Pat. No. 5,434,417 of David Nygren “Highresolution energy-sensitive digital X-ray” and US Patent ApplicationPublication 2004/0251419 of Robert Nelson. In US 2004/0251419, edge-ondetectors are used for so called Compton imaging, in which the energyand direction of the Compton scattered x-ray is measured in order tomake an estimation of the energy of the original x-ray. The method ofCompton imaging has been much discussed in the literature for a longtime but mainly applies to energies higher than what is employed inx-ray imaging, such as Positron Emission Tomography. Compton imagingdoes not relate to the present invention.

In a paper by S Shoichi Yoshida, Takashi Ohsugi “Application of siliconstrip detectors to X-ray computed tomography” Nuclear Instruments andMethods in Physics Research A 541 (2005) 412-420 an implementation ofthe edge-on concept is outlined. In this implementation thin tungstenplates placed between edge-on silicon strip detector reduces thebackground of scattered X-rays and improve the image contrast with lowdose. The implementation is very similar to what is proposed by R.Nowotny: “Application Of Si-Microstrip-Detectors In Medicine AndStructural Analysis” Nuclear Instruments and Methods in Physics Research226 (1984) 34-39.

Several proposals have been made for photon-counting semiconductordetectors based on high-Z materials such as CdZnTe and clinical imageshave also been acquired with prototype detectors. The drawback withthese materials is the cost and lack of experience in productionvolumes.

U.S. Pat. No. 8,183,535 discloses an example of a photon-countingedge-on x-ray detector. In this patent, there are multiple semiconductordetector modules arranged together to form an overall detector area,where each semiconductor detector module comprises an x-ray sensororiented edge-on to incoming x-rays and connected to integratedcircuitry for registration of x-rays interacting in the x-ray sensor.

The semiconductor detector modules are normally tiled together to form afull detector of almost arbitrary size with almost perfect geometricalefficiency except for an anti-scatter module which is integrated betweenat least some of the semiconductor detector modules. Preferably, eachanti-scatter module includes a foil of relatively heavy material toprevent most of the Compton scattered x-rays in a semiconductor detectormodule to reach an adjacent detector module.

FIG. 5 is a schematic diagram of an X-ray detector according to anexemplary embodiment. In this example there is shown a schematic view ofan X-ray detector (A) with x-ray source (B) emitting x-rays (C). Theelements of the detector (D) are pointing back to the source, and thuspreferably arranged in a slightly curved overall configuration. Twopossible scanning motions (E,F) of the detector are indicated. In eachscanning motion the source may be stationary or moving, in the scanningmotion indicated by (E) the x-ray source and detector may be rotatedaround an object positioned in between. In the scanning motion indicatedwith (F) the detector and the source may be translated relative to theobject, or the object may be moving. Also in scan motion (E) the objectmay be translated during the rotation, so called spiral scanning. By wayof example, for CT implementations, the x-ray source and detector may bemounted in a gantry that rotates around the object or subject to beimaged.

FIG. 6 is a schematic diagram illustrating an example of a semiconductordetector module according to an exemplary embodiment. This is an exampleof a semiconductor detector module with the sensor part 21 split intodetector elements or pixels 22, where each detector element (or pixel)is normally based on a diode having a charge collecting electrode as akey component. The x-rays enter through the edge of the semiconductorsensor.

FIG. 7 is a schematic diagram illustrating an example of semiconductordetector module according to another exemplary embodiment. In thisexample, the semiconductor sensor part 21 is also split into so-calleddepth segments 22 in the depth direction, again assuming the x-raysenter through the edge.

Normally, a detector element is an individual x-ray sensitivesub-element of the detector. In general, the photon interaction takesplace in a detector element and the thus generated charge is collectedby the corresponding electrode of the detector element.

Each detector element typically measures the incident x-ray flux as asequence of frames. A frame is the measured data during a specified timeinterval, called frame time.

Depending on the detector topology, a detector element may correspond toa pixel, especially when the detector is a flat-panel detector. Adepth-segmented detector may be regarded as having a number of detectorstrips, each strip having a number of depth segments. For such adepth-segmented detector, each depth segment may be regarded as anindividual detector element, especially if each of the depth segments isassociated with its own individual charge collecting electrode.

The detector strips of a depth-segmented detector normally correspond tothe pixels of an ordinary flat-panel detector. However, it is alsopossible to regard a depth-segmented detector as a three-dimensionalpixel array, where each pixel (sometimes referred to as a voxel)corresponds to an individual depth segment/detector element.

The semiconductor sensors may be implemented as so called Multi-ChipModules (MCMs) in the sense that the semiconductor sensors are used asbase substrates for electric routing and for a number of ApplicationSpecific Integrated Circuits (ASICs) which are attached preferablythrough so called flip-chip technique. The routing will include aconnection for the signal from each pixel or detector element to theASIC input as well as connections from the ASIC to external memoryand/or digital data processing. Power to the ASICs may be providedthrough similar routing taking into account the increase incross-section which is required for the large currents in theseconnections, but the power may also be provided through a separateconnection. The ASICS may be positioned on the side of the active sensorand this means it can be protected from the incident x-rays if anabsorbing cover is placed on top and it can also be protected fromscattered x-rays from the side by positioning an absorber also in thisdirection.

FIG. 8 is a schematic diagram illustrating an example of a semiconductordetector module. In this example, it is illustrated how the sensor area21 of the semiconductor detector (module) 20 also can have the functionof substrate in a Multi-Chip Module (MCM), similar to embodiments inU.S. Pat. No. 8,183,535. The signal is routed by signal paths 23 fromthe pixels 22 to inputs of parallel processing circuits 24 (e.g. ASICs)that are positioned next to the active sensor area. It should beunderstood that the term Application Specific Integrated Circuit (ASIC)is to be interpreted broadly as any general integrated circuit used andconfigured for a specific application. The ASICs process the electriccharge generated from each x-ray and converts it to digital data whichcan be used to detect a photon and/or estimate the energy of the photon.The ASICs may be configured for connection to digital processingcircuitry and/or memories located outside of the MCM and finally thedata will be used as input for reconstructing an image.

The proposed technology generally relates to a photon-counting detectorsystem with anti-coincidence logic that is controlled based on photoncount information, wherein the resulting count measurements are smoothlyvarying functions of the input count rate.

The inventors have recognized that the probability of false coincidenceincreases with increasing count rate, corresponding to higher incidentphoton flux. The probability of false coincidence also increases withthe range of the anti-coincidence logic, where the range is defined asthe maximum distance between pixels that allows the anti-coincidencelogic to identify pulses in the two pixels as belonging to the sameoriginal photon.

The proposed technology relates to a photon-counting x-ray detectorsystem comprising a plurality of photon-counting readout channels,wherein at least two of the channels are connected to an adaptiveanti-coincidence system configured to detect coincident events in theconnected channels, wherein the operation of the anti-coincidence logicis adapted or controlled based on photon count information, and whereinthe output of the detector system is a smoothly varying function of theinput count rate.

The anti-coincidence system is sometimes referred to as anti-coincidencelogic or anti-coincidence circuit.

By way of example, the operation of the anti-coincidence system may bevaried in dependence on photon count information. In other words, theanti-coincidence system may be configured to be operated differently independence on photon count information.

For example, the operation may be adapted to the photon flux rate, e.g.based on measured or estimated photon count rates or based on the numberof counts during a given period of time.

In other words, the photon count information may include informationrepresentative of an estimated photon count rate and/or informationrepresentative of the number of counts during a given period of time.

In a particular example, the operation of the anti-coincidence system isadapted in dependence on photon count information to be able to handledifferent incoming photon flux rates differently. In this way, arate-adaptive anti-coincidence system is obtained.

In a particular example, the anti-coincidence system may be selectivelyenabled and/or disabled based on the photon count information.

For example, the anti-coincidence logic may be enabled at low countrates and disabled at higher count rates. In this way, theanti-coincidence logic may be active and give improved DQE at low countrates where the probability of registering a false coincidence is low.At high count rates, the ACL may be inactive/deactivated/disabled andtherefore does not cause count loss due to false coincidence or degradeenergy resolution. At high count rates, double counting degrades DQE tosome extent since the anti-coincidence logic is not used. However, thisDQE loss is minor and preferable to the severe degradation that would becaused by count loss if the anti-coincidence logic were enabled. Thisdegradation of DQE on the noise level in the reconstructed image will ingeneral only have a minor impact on the quality of the reconstructedimage since the noise level in a CT image is dominated by the noisiestprojection lines, i.e. the projection lines with the lowest photon countrate. A degradation of DQE at high count rates therefore does not affectthe dominant contribution to the image noise.

The inventors have further recognized that an abrupt transition fromactive to inactive anti-coincidence logic may cause artifacts in theimage, since the output count rate then changes abruptly at a certaininput count rate, which can give rise to abrupt edges or streaks in thereconstructed image. By way of example, the anti-coincidence system maytherefore be disabled gradually at high or increasing count rates, aswill be exemplified later on. In other words, the transition from activeto inactive anti-coincidence logic with increasing count rate may begradual.

In a particular example, the anti-coincidence system may be implementedin a detector with multiple depth segments.

For example, the operation of the ACL in one depth segment of aconsidered detector pixel may be adapted in dependence on countinformation in another depth segment in the detector pixel, and/or countinformation in more than one depth segment in the detector pixel.

FIG. 9 is a schematic diagram, illustrating an example of aphoton-counting x-ray detector system having a plurality ofphoton-counting channels and at least one anti-coincidence circuit andan associated anti-coincidence controller according to an embodiment.

In this example, the photon-counting x-ray detector system 200 comprisesa plurality of photon-counting channels 220, and at least oneanti-coincidence circuit 230, each of which is connected to least two ofthe channels and configured to detect coincident events in the connectedchannels. The x-ray detector system further comprises ananti-coincidence controller 240 configured to control the operation ofsaid at least one anti-coincidence circuit 230 based on photon countinformation by gradually adapting the operation of said at least oneanti-coincidence circuit 230 with increasing count rates, starting froma threshold count rate.

Normally, each photon-counting channel 220 may be connected to acorresponding detector element 210, each of which typically has a chargecollecting electrode.

By way of example, the anti-coincidence controller 240 may be configuredto control the operation of said at least one anti-coincidence circuitby gradually disabling the anti-coincidence circuit(s) with increasingcount rates, starting from a threshold count rate.

For example, the anti-coincidence controller may be configured togradually disable the anti-coincidence circuit(s) with increasing countrates, starting from a threshold count rate and reaching a disabledstate at a second threshold rate.

In a particular example, the anti-coincidence circuit is configured todetect coincident events in the connected channels based on a set ofrules and/or settings relating to the pulse shape and time of incidence,wherein the set of rules and/or settings are gradually adapted withincreasing count rates.

For example, the anti-coincidence controller may be configured togradually adapt the set of rules and/or settings with increasing countrates to make the count-rate characteristic a smooth function.

As an example, the anti-coincidence controller may be configured togradually increase the fraction of time during which theanti-coincidence circuit is disabled with increasing count rates.

For example, the anti-coincidence controller may be configured togradually increase the fraction of time during which theanti-coincidence circuit is disabled per frame or per set of frames.

In a particular example, said at least one anti-coincidence circuit maybe enabled in at least one channel during at least part of at least oneframe and/or said at least one anti-coincidence circuit may be disabledin at least one channel in at least one frame.

Optionally, the anti-coincidence controller may be configured togradually decrease the maximum time separation between events thatallows the anti-coincidence circuit(s) to regard the events asoriginating from the same photon, with increasing count rates.

Alternatively, or as a complement, the anti-coincidence controller maybe configured to gradually lower the fraction of events processed by theanti-coincidence circuit(s) with increasing count rates.

In a particular example, the anti-coincidence controller is configuredto gradually reduce the neighborhood range of the anti-coincidencecircuit with increasing count rates.

As an example, the x-ray detector system comprises a plurality ofdetector elements, each connected to a corresponding photon-countingchannel, and the neighborhood range defines the maximum allowed distancebetween detector elements associated with connected channels of theanti-coincidence circuit.

A zero distance implies that the detector elements associated withconnected channels of the anti-coincidence circuit are adjacent to eachother, so-called nearest neighbors. A distance of one means that thedetector elements or channels are second-nearest neighbors, with onedetector element in-between, and so on.

Optionally, the anti-coincidence controller may be configured togradually alter the estimation of total deposited photon energy withincreasing count rates.

By way of example, the x-ray detector system 200 may comprise aplurality of detector elements 210, each connected to a correspondingphoton-counting channel.

In a particular example, the x-ray detector system may be based on adepth-segmented, edge-on x-ray detector, in which each detector strip issub-divided into at least two depth segments, each of which isconfigured as an individual detector element.

For example, a first anti-coincidence circuit connected to at least onedepth segment of at least one detector strip may be configured tooperate differently from a second anti-coincidence circuit connected toat least one other depth segment in the same detector strip, based onphoton count information.

Optionally, the operation of an anti-coincidence circuit for at leastone depth segment of at least one detector strip may be adapted orcontrolled based on photon count information of at least one other depthsegment in the same detector strip and/or based on photon countinformation in a plurality of depth segments belonging to the samedetector strip.

As an example, the photon count information may include informationrepresentative of an estimated photon count rate and/or informationrepresentative of the number of counts during a given period of time.

For example, the photon count information may be based on at least onecount rate parameter, which is calculated from previously measuredcounts in at least one channel.

In a particular example, the anti-coincidence controller 240 isconfigured to gradually adapt the operation of an anti-coincidencecircuit for a number of connected channels based on photon countinformation related to at least one other channel separate from theconnected channels.

Preferably, said at least one anti-coincidence circuit may be configuredto identify the channel of the original photon interaction and/orestimate the total energy of the original photon.

The proposed technology also relates to an anti-coincidence system 250for a photon-counting x-ray detector system 200 having a plurality ofphoton-counting readout channels 220. The anti-coincidence system 250comprises at least one anti-coincidence circuit 230, each of which isconnected to least two of the channels and configured to detectcoincident events in the connected channels. The anti-coincidence system250 further comprises an anti-coincidence controller 240 configured tocontrol the operation of said at least one anti-coincidence circuit 230based on photon count information by gradually adapting the operation ofsaid at least one anti-coincidence circuit with increasing count rates,starting from a threshold count rate.

In another aspect, the proposed technology also relates to a controller240 for an anti-coincidence system 250 of a photon-counting x-raydetector system having a plurality of photon-counting readout channels220. The anti-coincidence system 250 comprises at least oneanti-coincidence circuit 230, each of which is connected to least two ofthe channels and configured to detect coincident events in the connectedchannels. The controller 240 is configured to control the operation ofsaid at least one anti-coincidence circuit 230 based on photon countinformation by gradually adapting the operation of said at least oneanti-coincidence circuit with increasing count rates, starting from athreshold count rate.

For a better understanding of certain aspects, the proposed technologywill now be described with reference to particular, non-limitingexamples.

FIG. 10 is a schematic drawing illustrating another example of ananti-coincidence system implemented in a photon-counting x-ray detectorsystem.

The signal value in each analog input signal channel is compared to oneor more comparators, which compare the signal level to a predefined setof voltage thresholds. The digital output signal from the set ofcomparator(s) belonging to one analog input channel is transmitted to apriority decoder, which translates the set of comparator trigger signalsto a digital value containing the number of the triggering comparatorthat corresponds to the highest threshold. The result is a digital valueindicating the height of the pulse relative to the comparator thresholdlevels, and it will be referred to as a pulse-height value.

The pulse-height values from the priority decoders are transmitted toanti-coincidence circuits with the purpose of detecting and correctingfor coincident pulses. In one embodiment of the invention, theanti-coincidence logic is used in a one-dimensional implementation. Inthis case, for every pair of neighboring detector channels ananti-coincidence circuit is provided, which receives the outputpulse-height values from both channels in said pair of detectorchannels. Here and from now on, “neighboring channels” refers tochannels which are connected to detector elements which are located nextto each other, but may also be located in the same neighborhood althoughnot adjacent to each other such as second-nearest neighbors.

In general, the photon interaction takes place in a detector element andthe thus generated charge is collected by the corresponding electrode(s)of the detector element. Normally, a detector element is made up by adetector diode which has corresponding electrode(s).

In a preferred embodiment of the invention, the anti-coincidence circuitis programmed with a pre-configured time window setting, and detects iftwo pulses arrive in the two neighboring channels with a time differenceless than or equal to this time delay. Using the information containedin the input signals from the two channels, such as the pulse-heightvalues and the relative timing of the pulses, the anti-coincidencecircuit determines whether the detected pulses come from the sameoriginal incident photon or not. If the pulses are identified asoriginating from the same photon, the anti-coincidence circuit assignsthe event to one of the two neighboring channels and estimates the pulseheight corresponding to the total energy deposited by the photon. Such acorrection may take the form of a look-up table with two input signals:the pulse-height signals in each of the two neighboring channels, andone output signal: the estimated pulse-height signal corresponding tothe total energy. Alternatively, the correction may take the form of amathematical expression relating the output signal to the input signals,e.g. by letting the output be the sum of the input signals.

The anti-coincidence circuits may use a set of rules and/or settings todetermine whether the event is a true or false coincidence, i.e. whetherthe detected pulses originate from the same photon or not. In oneembodiment, these rules and/or settings may include comparing the sum ofthe two pulse heights to a predefined limit, classifying the event as atrue coincidence if the sum is smaller than the limit and as a truecoincidence otherwise. In another embodiment, these rules may includecomparing the arrival times and pulse height of two detected pulses andclassifying the event as a false coincidence if the pulse with largerpulse height arrived before the pulse with smaller height, andclassifying it as a true coincidence otherwise. The anti-coincidencecircuits may also use a set of rules to assign the interaction to achannel, where the original interaction is estimated to have takenplace. In one embodiment, these rules may include assigning the event tothe channel where the largest pulse height was detected. In anotherembodiment, the rules may include assigning the event to the channelwhere the pulse arrived earliest.

Furthermore, for each input signal channel there is a counter bank thatincludes one or more counters. The pulse-height signal for that signalchannel, together with the output signals from the anti-coincidencecircuits of the two neighbor pairs that the channel belongs to, aretransmitted to the counter bank. Based on these signals, one or morecounters in the counter bank are incremented. In a preferred embodiment,for every comparator there is a counter in the counter bank that, ifthere are no coincident events, counts the number of incident pulses inthe channel with a pulse height between the threshold level of saidcomparator and the threshold level of the following comparator in orderof increasing threshold. In case there are coincident events detected onneighboring channels, the output of the anti-coincidence circuit ismodified accordingly, so that only one counter is incremented for everyregistered photon and so that the incremented counter is the onecorresponding to the estimated total deposited photon energy.Specifically, upon arrival of coincident pulses on two neighboringchannels, the event will be registered in one of the channels, namelythe one that the event is assigned to by the anti-coincidence circuitcorresponding to these two channels. Also, the output of the logic, e.g.a lookup table, gives the number of the counter that is incremented inthe counter bank of that channel.

The above description typically relates to an implementation where theanti-coincidence correction is performed in one dimension. Such animplementation is useful in a detector design where charge sharing andother effects causing cross-talk between detector elements takes placepredominantly in one dimension. Examples of such detectors includesilicon strip detectors oriented with their edge directed towards thebeam. In such a detector, the charge-sharing may take placepredominantly in one dimension even if the detector is depth-segmented,if the depth segment lengths are large compared to the charge carriercloud size. In this case, an anti-coincidence circuit is includedbetween neighboring detector elements in the same depth segment level.The anti-coincidence logic may also be implemented in a two-dimensionalgeometry by including an anti-coincidence circuit between any/every pairof neighboring pixels sharing a common pixel border.

The anti-coincidence logic can also be implemented in other ways, forexample by also including anti-coincidence circuits between pixelssharing a common corner, and potentially also between pixels separatedeven further apart. A similar scheme can also be implemented in athree-dimensional pixel array, such as a depth-segmented detector, byimplementing anti-coincidence circuits between neighboring/adjacentdetector elements in all three dimensions. In another exemplaryembodiment, the anti-coincidence circuits may receive pulse-heightsignals from three or more channels, so that events occurringsimultaneously on three or more channels may be detected and corrected.

In another embodiment of the invention, the anti-coincidence circuitsare directly connected to the comparator output signals, withoutpriority decoders in between. In yet another embodiment of theinvention, the anti-coincidence logic is implemented in such a way thatthe anti-coincidence circuits use the analog signals directly as inputs,without comparators and priority decoders in between. This allows theanti-coincidence circuits to use an analog summation circuit tocalculate the total pulse height as the sum of the pulse heights of thedifferent channels that detect pulses.

In one aspect of the invention, the detector logic contains ananti-coincidence control unit that senses the count rate in each frameand adapts the operation of the anti-coincidence logic as a function ofone or more count information parameters. Such a count informationparameter may be either individual for each readout channel or commonfor a plurality of readout channels, where in the latter case the totalcounts in all the affected readout channels may be used to control theoperation of the anti-coincidence logic.

The output count rate measured by the detector is the number ofregistered counts per frame divided by the frame time. This may bedifferent from the input count rate, i.e. the actual rate ofinteractions in the detector material. The relationship between theexpectation value of the input count rate and the expectation value ofthe output count rate in a photon-counting detector is referred to as acount-rate characteristic. The term “expectation value” is here used inthe sense of probability theory, i.e. the input and output count ratesare viewed as random variables. In the presence of pile-up, thecount-rate characteristic is non-linear.

FIG. 11A shows the count-rate characteristic for a detector with andwithout anti-coincidence logic. The anti-coincidence logic decreases thecount rate by removing coincident counts and therefore decreases theoutput count rate for a given input count rate.

If the anti-coincidence logic is disabled abruptly, at a certain countrate, the count rate characteristic will be discontinuous, as shown inFIG. 11B. At said count rate, the input-output characteristic jumps fromthe lower curve, which applies to a detector with anti-coincidencelogic, to the upper curve, which applies to a detector withoutanti-coincidence logic. Such a rapid change in detector response maygive rise to artifacts in the image.

To reduce artifacts, it is desirable to have a count-rate characteristicthat is smoothly varying. In general, it is desirable that all countvalues measured by the detector are smoothly varying functions of theinput count rate. In this context, a smooth function, or a smoothlyvarying function, is a function whose output variable does not vary toorapidly with the input variable, with respect to the intendedapplication. The requirements for a function to be smooth may thus bedifferent in different applications.

By way of example, a count-rate characteristic may be regarded as smoothif it is a continuous function. In another example, a count-ratecharacteristic may be regarded as smooth if it is a differentiablefunction. In yet another example, a count-rate characteristic may beregarded as smooth even if it is discontinuous at a number of points,provided that the discontinuities are small enough not to give visibleartifacts in the image.

In a particular example of the present invention, a method is providedfor disabling the anti-coincidence logic gradually within a range ofcount rates. More specifically, the fraction of coincident eventsremoved by the anti-coincidence logic is reduced to zero gradually asthe count rate increases, thereby obtaining a detector whoseinput-output count rate characteristic is smooth.

An example of such a count-rate characteristic is shown in FIG. 11C. Inan exemplary embodiment, the anti-coincidence logic is enabled duringthe entirety of each frame for output count rates lower than a firstthreshold count rate m₁. For output count rates higher than the firstthreshold count rate m₁ and lower than a second threshold count rate m₂,the anti-coincidence logic is disabled during part of each frame andenabled during the rest of the frame. Between output count rates m₁ andm₂, the fraction of each frame during which the anti-coincidence logicis disabled is an increasing function of the output count rate. In thisexample, for count rates larger than the second threshold count rate m₂,the anti-coincidence logic is constantly kept in a disabled state.

U.S. Pat. No. 9,207,332 relates to a specialized detector configurationwith two modes, one where the charges are summed prior to digitizationand one where the counts is summed after digitization.

US Patent Application 2016/0282476 relates to an x-ray detector with twocounting modes, where a first count measurement in a first counting modeis used to control a second count measurement in a second counting mode.

Neither of these two examples of prior art discloses an anti-coincidencesystem that is turned off gradually with increasing count rate. Also,neither of these two examples of prior art discloses an anti-coincidencelogic that may be adapted for particular use with a depth-segmenteddetector.

In another embodiment of the invention, the anti-coincidence logic maybe disabled during a fraction ƒ of the frame given by a mathematicalfunction ƒ(m) of the output count rate m. In a preferred embodiment,this function may be smooth and monotonically increasing from 0 to 1,for example

${f(n)} = {0.5 \cdot \left( {1 + {{erf}\frac{m - m_{t}}{\sigma \sqrt{2}}}} \right)}$

where nm_(t) is a threshold output count rate and σ is a parametercontrolling the transition smoothness. In another embodiment, thefunction ƒ(m) may assume values between ƒ₁ and ƒ₂ for all count rates,where one of the conditions ƒ₁>0 and ƒ₂<1 or both may be true, so thatthe anti-coincidence logic is either enabled during part of the frametime for all count rates, or always disabled during part of the countrate, or both.

In another embodiment of the invention, the anti-coincidence logic isenabled during a subset of the acquired frames and disabled during theother frames, where said subset of frames depends on the count rate. Forexample, the anti-coincidence logic can be enabled during every frame atlow count rates, enabled during every second frame at intermediate countrates and disabled during every frame at high count rates.

In yet another embodiment of the invention, the measured count rate isaveraged over several frames before being used for controlling theanti-coincidence logic. This may be implemented by continuouslycalculating a moving average of a specified number of the most recentlyacquired frames. This moving average is then taken as input by theanti-coincidence logic control circuit in order to determine when theanti-coincidence logic should be enabled and when it should be disabled.In this way, the effect of statistical fluctuations in the measuredcount rate on the anti-coincidence logic is reduced.

In still another embodiment of the invention, a counter is provided foreach anti-coincidence circuit that counts the number of truecoincidences that has been detected in that anti-coincidence circuit.Once said counter reaches a certain threshold value, saidanti-coincidence logic pair is disabled so that it no longer correctsfor coincidence events. With the start of each frame, the counter isreset so that the anti-coincidence circuit is enabled again until thecounter reaches the threshold value. In this way, the anti-coincidencelogic will be active for the entire frame for low count rates but onlyduring a fraction of the frame for high count rates. The threshold mayeither be a fixed pre-programmed value or a function of the incidentcount rate.

In another embodiment of the invention, the anti-coincidence may or maynot process each event, depending on a random selection with a certainprobability, where said probability is varied with photon countinformation such as the count rate or the number of previouslyregistered count within a certain time period.

In another aspect of the invention, the time window length of theanti-coincidence logic, which determines the maximum time separationbetween events that allows the anti-coincidence logic circuit to regardthem as originating from the same photon, is varied depending on themeasured incident count rate. In one embodiment of the invention, thetime window length is equal to a first value t₁ for output count ratesless than m₁ and a second value t₂ for output count rates greater thanm₂. Between m₁ and m₂ the time window is equal to t₁ for a fraction ƒ ofeach frame and equal to t₂ for the remainder of each frame, where ƒincreases smoothly from 0 to 1 between output count rates m₁ and m₂. Forexample t₂ can be chosen less than t₁, which means that the time windowdecreases with increasing output count rate. Decreasing the time windowlength decreases the false coincidence rate, meaning that the schemedescribed here mitigates the DQE degradation due to false coincidence athigh count rates.

In yet another aspect of the invention, the anti-coincidence logic isendowed with at least one range parameter that affects the range of theanti-coincidence logic, and said range parameter(s) are taken to befunctions of the input count rate. In one embodiment, theanti-coincidence logic may act on nearest-neighboring andsecond-nearest-neighboring channel pairs (i.e. channel pairs connectedto electrodes with at most one electrode in between them) for low photonfluxes, but only on nearest-neighboring electrodes for high photonfluxes. For intermediate photon fluxes the anti-coincidence logic actson nearest-neighbor and second-nearest-neighbor pairs during part ofeach frame and only on nearest-neighbor frames during the rest of eachframe.

In still another aspect of the invention, the algorithm for estimatingthe total deposited energy may be gradually altered as a function of thecount rate, e.g. gradually altered with increasing count rates.Specifically, if a look-up table is used to generate an outputpulse-height value from at least two input pulse-height values, thevalues in said look-up table may be taken to be functions of depositedenergy. This allows adjusting the look-up table to reproduce theestimated total photon energy more accurately for a range of differentcount rates. In particular, this may mitigate the detrimental effects ofspectral distortion caused by pile-up on the function of theanti-coincidence logic. At high count rates, pile-up may causedistortion of the detected energy spectrum, since the total height oftwo pulses arriving simultaneously in the same channel is measured. Bytaking this into account in the estimation of original photon energy inthe anti-coincidence logic circuit, the initial photon energy may bereproduced more accurately.

In another aspect of the invention, the anti-coincidence logic connectedto one detector pixel may be controlled or adapted based on countinformation in one or more other detector pixels. For example, theanti-coincidence logic connected to a first detector pixel may beadapted based on the count rate in neighboring pixels, for example tostart a gradual reduction of the anti-coincidence system whenneighboring detector pixels reach a certain threshold count rate eventhough the first detector pixel has not reached said threshold countrate. This may allow the count-rate characteristic to become smoother.

In another aspect of the invention, the anti-coincidence logic systemmay be used together with a depth-segmented detector, in which eachdetector strip (pixel) is subdivided into a plurality of depth segmentsalong the intended x-ray beam direction. By way of example, theoperation of the anti-coincidence logic may be different in thedifferent depth segments. In particular, the anti-coincidence logic maybe adapted by letting it be turned on for a fraction of each frame,where said fraction is a function of count rate, as previouslydiscussed. In this case, the anti-coincidence logic may be active duringdifferent frame times in different depth segments. Since the total countnumber detected in the detector pixel is the sum of the count number inthe depth segments, this makes the total number of counts in all depthsegments a smoother function of input count rate than what would havebeen possible with only one depth segment. In situations where a smoothdecrease in the anti-coincidence logic activity is hard to implement,for example in systems where the time fraction during which theanti-coincidence logic system is active can only be set to a discretenumber of values.

In another aspect of the invention, the usage of a depth-segmenteddetector can also be beneficial in other ways for the disclosedanti-coincidence logic system. For example, the operation of ananti-coincidence circuit for at least one depth segment of at least onedetector strip may be adapted or controlled based on photon countinformation of at least one other depth segment in the same detectorstrip and/or based on photon count information in a plurality of depthsegments belonging to the same detector strip. In other words, the countinformation obtained in one or more depth segments of one detectorstrip/pixel may be used to control or adapt anti-coincidence logicconnected to one of the depth segments, or to a subset of the depthsegments, or in another depth segment of the same detector strip/pixel.For example, the total number of counts registered in all depth segmentsof one detector pixel may be used to control the anti-coincidence logicconnected to one of the depth segments in the same detector pixel. Inthis way, more photon statistics is available to the anti-coincidencecontrol logic and the influence of quantum noise on the anti-coincidenceoperation is thereby decreased. This reduces the risk that theanti-coincidence logic is turned on and off randomly because of quantumnoise.

In another example, the anti-coincidence logic connected to one depthsegment which measures lower count rate in general may be adapted basedupon one or more other depth segments in the same detector strip/pixelwhich measure higher count rates, thereby reducing the influence ofquantum noise.

In another example, the anti-coincidence logic connected to one depthsegment which measures higher count rate in general may be adapted basedupon one or more other depth segments in the same detector strip/pixelwhich measure lower count rates. In this way, the anti-coincidence logicmay for example be controlled based on the count rate in a depth segmentin which the anti-coincidence logic is constantly turned on in therelevant range of photon flux rates. In this way, the rapid change inoutput count rate when the anti-coincidence logic is turned off orswitches from one mode to another does not feed back into theanti-coincidence control logic, where it could otherwise complicate thecontrol algorithm.

It will be appreciated that the mechanisms and arrangements describedherein can be implemented, combined and re-arranged in a variety ofways.

For example, embodiments may be implemented in hardware, or at leastpartly in software for execution by suitable processing circuitry, or acombination thereof.

The steps, functions, procedures, and/or blocks described herein may beimplemented in hardware using any conventional technology, such asdiscrete circuit or integrated circuit technology, including bothgeneral-purpose electronic circuitry and application-specific circuitry.

Alternatively, or as a complement, at least some of the steps,functions, procedures, and/or blocks described herein may be implementedin software such as a computer program for execution by suitableprocessing circuitry such as one or more processors or processing units.

FIG. 12 is a schematic diagram illustrating an example of acomputer-implementation 300 according to an embodiment. In thisparticular example, at least some of the steps, functions, procedures,modules and/or blocks described herein are implemented in a computerprogram 325; 335, which may be loaded from an external memory device 330into the memory 320 for execution by processing circuitry including oneor more processors 310. The processor(s) 310 and memory 320 areinterconnected to each other to enable normal software execution. Anoptional input/output device 340 may also be interconnected to theprocessor(s) 310 and/or the memory 320 to enable input and/or output ofrelevant data such as input parameter(s) and/or resulting outputparameter(s).

The term ‘processor’ should be interpreted in a general sense as anysystem or device capable of executing program code or computer programinstructions to perform a particular processing, determining orcomputing task.

Examples of processing circuitry includes, but is not limited to, one ormore microprocessors, one or more Digital Signal Processors (DSPs), oneor more Central Processing Units (CPUs), and/or any suitableprogrammable logic circuitry such as one or more Field Programmable GateArrays (FPGAs), or one or more Programmable Logic Controllers (PLCs).

The processing circuitry including one or more processors 310 is thusconfigured to perform, when executing the computer program 325,well-defined processing tasks such as those described herein.

By way of example, the software or computer program 225; 235 may berealized as a computer program product, which is normally carried orstored on a computer-readable medium 220; 230, in particular anon-volatile medium. The computer-readable medium may include one ormore removable or non-removable memory devices including, but notlimited to a Read-Only Memory (ROM), a Random Access Memory (RAM), aCompact Disc (CD), a Digital Versatile Disc (DVD), a Blu-ray disc, aUniversal Serial Bus (USB) memory, a Hard Disk Drive (HDD) storagedevice, a flash memory, a magnetic tape, or any other conventionalmemory device. The computer program may thus be loaded into theoperating memory of a computer or equivalent processing device forexecution by the processing circuitry thereof.

More specifically, the computer-program product comprises acomputer-readable medium 320, 330 having stored thereon a computerprogram 325, 335 for controlling, when executed by a processor, ananti-coincidence system of a photon-counting x-ray detector systemhaving a plurality of photon-counting readout channels, wherein theanti-coincidence system comprises at least one anti-coincidence circuit,each of which is connected to least two of the channels and configuredto detect coincident events in the connected channels. The computerprogram comprises instructions, which when executed by the processor,cause the processor to control the operation of said at least oneanti-coincidence circuit based on photon count information by graduallyadapting the operation of said at least one anti-coincidence circuitwith increasing count rates, starting from a threshold count rate.

It should also be understood that it may be possible to re-use thegeneral processing capabilities of any conventional device or unit inwhich the proposed technology is implemented. It may also be possible tore-use existing software, e.g. by reprogramming of the existing softwareor by adding new software components.

The extent of software versus hardware is purely an implementationselection.

The embodiments described above are merely given as examples, and itshould be understood that the proposed technology is not limitedthereto. It will be understood by those skilled in the art that variousmodifications, combinations and changes may be made to the embodimentswithout departing from the present scope as defined by the appendedclaims. In particular, different part solutions in the differentembodiments can be combined in other configurations, where technicallypossible.

1. A photon-counting x-ray detector system comprising: a plurality ofphoton-counting channels, and at least one anti-coincidence circuit,each of which is connected to least two of the channels and configuredto detect coincident events in the connected channels, wherein the x-raydetector system further comprises an anti-coincidence controllerconfigured to control the operation of said at least oneanti-coincidence circuit based on photon count information by graduallyadapting the operation of said at least one anti-coincidence circuitwith increasing count rates, starting from a threshold count rate. 2.The photon-counting x-ray detector system of claim 1, wherein theanti-coincidence controller is configured to control the operation ofsaid at least one anti-coincidence circuit by gradually limiting theinfluence of the at least one anti-coincidence circuit with increasingcount rates, starting from a threshold count rate.
 3. Thephoton-counting x-ray detector system of claim 2, wherein theanti-coincidence controller is configured to gradually limit theinfluence of the at least one anti-coincidence circuit with increasingcount rates to make the count rate characteristic a continuous function.4. The photon-counting x-ray detector system of claim 1, wherein theanti-coincidence circuit is configured to detect coincident events inthe connected channels based on a set of rules and/or settings relatingto the pulse shape and time of incidence, wherein the set of rulesand/or settings are gradually adapted with increasing count rates. 5.The photon-counting x-ray detector system of claim 1, wherein theanti-coincidence controller is configured to operate based on a maximumtime separation between events that allows the at least oneanti-coincidence circuit to regard the events as originating from thesame photon, and to gradually decrease the maximum time separation withincreasing count rates.
 6. The photon-counting x-ray detector system ofclaim 1, wherein the anti-coincidence controller is configured togradually lower the fraction of events processed by the at least oneanti-coincidence circuit with increasing count rates.
 7. Thephoton-counting x-ray detector system of claim 1, wherein the x-raydetector system comprises a plurality of detector elements, eachconnected to a corresponding photon-counting channel, and theanti-coincidence controller is configured to gradually reduce a maximumallowed distance between detector elements associated with connectedchannels of the at least one anti-coincidence circuit with increasingcount rates.
 8. The photon-counting x-ray detector system of claim 1,wherein the anti-coincidence controller is configured to gradually alterthe estimation of total deposited photon energy with increasing countrates.
 9. The photon-counting x-ray detector system of claim 1, whereinthe x-ray detector system comprises a plurality of detector elements,each connected to a corresponding photon-counting channel.
 10. Thephoton-counting x-ray detector system of claim 9, wherein the x-raydetector system is based on a depth-segmented, edge-on x-ray detector,in which each detector strip is sub-divided into at least two depthsegments, each of which is configured as an individual detector element.11. The photon-counting x-ray detector system of claim 10, wherein afirst anti-coincidence circuit connected to at least one depth segmentof at least one detector strip is configured to operate differently froma second anti-coincidence circuit connected to at least one other depthsegment in the same detector strip, based on photon count information.12. The photon-counting x-ray detector system of claim 10, wherein theoperation of an anti-coincidence circuit for at least one depth segmentof at least one detector strip is adapted or controlled based on photoncount information of at least one other depth segment in the samedetector strip and/or based on photon count information in a pluralityof depth segments belonging to the same detector strip.
 13. Thephoton-counting x-ray detector system of claim 1, wherein said photoncount information includes information representative of an estimatedphoton count rate and/or information representative of the number ofcounts during a given period of time.
 14. The photon-counting x-raydetector system of claim 13, wherein the photon count information isbased on at least one count rate parameter, which is calculated frompreviously measured counts in at least one channel.
 15. Thephoton-counting x-ray detector system of claim 1, wherein theanti-coincidence controller is configured to gradually adapt theoperation of an anti-coincidence circuit for a number of connectedchannels based on photon count information related to at least one otherchannel separate from the connected channels.
 16. The photon-countingx-ray detector system of claim 1, wherein said at least oneanti-coincidence circuit is configured to identify the channel of theoriginal photon interaction and/or estimate the total energy of theoriginal photon.
 17. An anti-coincidence system for a photon-countingx-ray detector system having a plurality of photon-counting readoutchannels, wherein the anti-coincidence system comprises at least oneanti-coincidence circuit, each of which is connected to least two of thechannels and configured to detect coincident events in the connectedchannels, wherein the anti-coincidence system further comprises ananti-coincidence controller configured to control the operation of saidat least one anti-coincidence circuit based on photon count informationby gradually adapting the operation of said at least oneanti-coincidence circuit with increasing count rates, starting from athreshold count rate.
 18. A controller for an anti-coincidence system ofa photon-counting x-ray detector system having a plurality ofphoton-counting readout channels, wherein the anti-coincidence systemcomprises at least one anti-coincidence circuit, each of which isconnected to least two of the channels and configured to detectcoincident events in the connected channels, wherein the controller isconfigured to control the operation of said at least oneanti-coincidence circuit based on photon count information by graduallyadapting the operation of said at least one anti-coincidence circuitwith increasing count rates, starting from a threshold count rate.
 19. Anon-transitory computer-readable medium having stored thereon a computerprogram for controlling, when executed by a processor, ananti-coincidence system of a photon-counting x-ray detector systemhaving a plurality of photon-counting readout channels, wherein theanti-coincidence system comprises at least one anti-coincidence circuit,each of which is connected to least two of the channels and configuredto detect coincident events in the connected channels, wherein thecomputer program comprises instructions, which when executed by theprocessor, cause the processor to control the operation of said at leastone anti-coincidence circuit based on photon count information bygradually adapting the operation of said at least one anti-coincidencecircuit with increasing count rates, starting from a threshold countrate.